1. Field of the Invention
The present invention relates to an image forming apparatus that charges the surface of a photoconductor by using a charging device. More particularly, the present invention relates to an image forming apparatus that uses a scorotron having a grid between a corona wire and an article to be charged.
2. Description of Related Art
FIG. 6 shows the structure of a conventional image forming apparatus. Inside an image forming apparatus 1, an image formation portion P is located above a transportation belt 8. This image formation portion P forms a predetermined image through the processes of charging, exposure, development, and transfer.
The image formation portion P includes a photoconductive drum 2 that carries a visible image (a toner image), and is so constructed that a toner image formed on the photoconductive drum 2 is transferred onto a sheet (a recording medium) 6 supported/transported by the transportation belt 8 that moves adjacent to the image formation portion P, then is fixed to the sheet 6 by a fixing portion 7, and then is discharged from an apparatus main body. In FIG. 6, an image formation process is performed on the photoconductive drum 2 while rotating the photoconductive drum 2 in clockwise direction.
Next, the image formation portion P will be described in detail. There are disposed, around and above the photoconductive drum 2 that is rotatably mounted, a charging device (a charger) 3 that charges the photoconductive drum 2, an exposure unit 4 that exposes image information onto the photoconductive drum 2, a developer unit 5 that forms a toner image on the photoconductive drum 2, and a cleaning unit 9 that removes the developer (toner) remaining on the photoconductive drum 2.
First, the entire surface of the photoconductive drum 2 is uniformly charged by the charging device 3, and then the charged surface thereof is irradiated with light shone from the exposure unit 4 so as to form an electrostatic latent image on the photoconductive drum 2 according to an image signal. A toner container 10 fills the developer unit 5 with a predetermined amount of toner. The toner is supplied from the developer unit 5 to the photoconductive drum 2, and is then electrostatically attached thereto, whereby a toner image in accordance with the electrostatic latent image formed by exposure performed by the exposure unit 4 is formed thereon.
The sheet 6 onto which the toner image is transferred is accommodated in a plurality of paper feed cassettes 11a, 11b, and 11c that accommodate paper sheets, and a stack bypass (a manual feed tray) 11d located above them. The sheet 6 is fed onto the transportation belt 8 via paper feed rollers 12 and resist rollers 13, and is then transported to a position at which the photoconductive drum 2 is located. Used as the transportation belt 8 is a dielectric resin sheet that is formed into an endless belt by bonding the two ends of the sheet together or into a seamless belt.
The transportation belt 8 is stretched taut between a drive roller 14 disposed most downstream and a driven roller 15 disposed most upstream. When the transportation belt 8 starts to rotate counterclockwise, the sheet 6 is transported from the resist roller 13 onto the transportation belt 8. At this time, an image writing signal turns on, and an image is formed on the photoconductive drum 2 with predetermined timing. Then, in the electric field produced by a transfer roller 16 provided below the photoconductive drum 2 and having a predetermined transfer voltage applied thereto, the toner image on the photoconductive drum 2 is transferred onto the sheet 6. The sheet 6 is electrostatically adsorbed onto the transportation belt 8.
The sheet 6, having the toner image transferred thereon, then leaves the transportation belt 8, and is then transported to the fixing portion 7. After the transfer of the toner image, the cleaning unit 9 cleans the photoconductive drum 2, having the toner image transferred therefrom, to remove the toner remaining on the surface thereof in preparation for the formation of new electrostatic latent images. The sheet 6 transported from the transportation belt 8 to the fixing portion 7 is subjected to application of heat and pressure by a fixing roller 7a so as to fix the toner image to the surface of the sheet 6. In this way, a predetermined image is formed. The sheet 6 on which the image is formed is discharged to an output tray 18 by an output roller 17.
The charging device 3 used in the image forming apparatus described above has a corona wire (a charge wire) that is a thin wire made of tungsten or stainless steel. When a high voltage is applied to the corona wire electrode, discharge (corona discharge) takes place, whereby the charging device 3 charges the surface of the photoconductive drum 2. Two types of known charging devices are scorotrons having a grid electrode between the corona wire and an article to be charged, and corotrons having no grid electrode. Scorotrons are superior to corotrons in that they can perform charging control by varying the voltage applied to the grid.
FIG. 7 shows the structure of a scorotron. The charging device 3 is disposed in the width direction of the photoconductive drum 2 (in the direction perpendicular to the plane of FIG. 7), and is provided with a shield member (a casing) 31 with a C-shaped cross section having an open portion 31a on the surface side of the photoconductive drum 2, a corona wire 32 to which a high voltage is applied, and a grid 33 provided in the open portion 31a. The charging device 3 makes the surface of the photoconductive drum 2 charged to a predetermined positive potential via the grid 33 by corona discharge from the corona wire 32. However, if the grid 33 is stained with toner or the like, charging becomes unstable, resulting in degradation in image quality such as uneven density of the image at the time of image formation.
On the other hand, some elements in the air are oxidized by ozone produced by corona discharge, leading to the formation of an ion product such as NOx or SOx. The ion product thus formed and dust in the air settle on the shield member 31 and the grid 33. In general, these extraneous matters serve as insulation, and adversely affect charging characteristics of the charging device 3. Above all, when these matters adhere to the grid 33, a current supposed to flow into the grid 33 actually flows out of a grid opening into the photoconductive drum 2, leading to significant increase in the surface potential of the photoconductor. This causes problems such as a decrease in image density. Therefore, it is necessary to make the user or repair people clean the grid 33 on a regular basis or when image quality degrades, or to replace the grid 33 alone or the entire charging device 3 including the grid 33. This increases costs and trouble.
In view of the problems described above, a method of cleaning the grid surface with a simple structure is proposed. Japanese Patent Application Laid-Open No. H9-197771 (hereinafter referred to as Patent Publication 1) discloses a method of cleaning a grid and a corona wire at the same time by making a grid cleaner having a wet sponge slide along a charging device. The ion product mentioned above is soluble in water, and therefore it is possible to easily remove contaminants strongly adhered to the grid 33 by using a grid cleaner having a wet sponge.
FIG. 8 is a partially enlarged view showing how the charging device of the conventional image forming apparatus is cleaned. When degradation of image quality is observed, or when a periodical inspection is conducted, the charging device 3 is cleaned as follows. The user first opens a front cover 34 of the apparatus main body, then takes off a storage container (not shown) for storing the recovered toner, and then fits a grid cleaner 35 to a fixing holder (not shown) provided in the vicinity of an extraction opening 36 through which the charging device 3 is pulled out.
The grid cleaner 35 is included with a spare toner container or a maintenance kit. As shown in FIG. 9, the grid container 35 is composed of a resin case 37 and a wet sponge 38 provided so as to project from a floor face 37a of the case 37. The wet sponge 38 is impregnated with water or alcohol-containing water. The grid cleaner 35 is sealed with a moisture impermeable film or the like until the point of use to prevent the wet sponge 38 from drying out. Reference numeral 37b denotes an insertion portion that is inserted into the above-described fixing holder for fixing purposes.
Now, the description of the charging device 3 shown in FIG. 8 will be continued. By sliding the charging device 3 with the grid cleaner 35 fixed thereto, the user pulls the charging device 3 out of the apparatus main body, and then inserts it thereinto. The user repeats this, whereby the grid 33 (see FIG. 7) of the charging device 3 is rubbed with the wet sponge 38 of the grid cleaner 35, and the extraneous matters on the surface of the grid 33 are removed therefrom. Between the charging device 3 and the extraction opening 36, a slight clearance (play) is secured so that the charging device 3 smoothly slides regardless of irregularities on the side of the grid 33.
FIG. 10 is a perspective view showing an apparatus front side end portion of the charging device 3, as seen from the grid 33 side. In the following description, such members as are found also in FIG. 7 will be identified with common reference numerals and their descriptions will be omitted. Reference numeral 39 denotes a gap roller that maintains the distance between the photoconductive drum 2 and the grid 33 constant and that is so disposed as to project from an opening 33a formed in the grid 33. The charging device 3 has, in an apparatus back side end portion thereof, another gap roller 39 having the same structure. Reference numeral 40 denotes stopper projections that prevent the charging device 3 from being disconnected from the apparatus main body at times other than during cleaning.
Next, a positional relationship between the grid and the grid cleaner as observed when the charging device 3 is pulled out during cleaning will be described with reference to FIG. 11. By inserting the insertion portion 37b into the fixing holder 41, the grid cleaner 35 is fixed to an apparatus main body side in such a way that the wet sponge 38 faces the grid 33. When the charging device 3 is made to slide from its standard position shown in FIG. 11A in the direction indicated by arrow A shown in the figure, the wet sponge 38 is rubbed with the gap roller 39 when the gap roller 39 passes by the grid cleaner 35 as shown in FIG. 11B. Then, as shown in FIG. 11C, the wet sponge 38 makes contact with the surface of the grid 33, whereby the extraneous matters are removed therefrom.
The problem here is that, when the method disclosed in Patent Publication 1 is adopted, water squeezed out of the wet sponge 38 when it is rubbed against the gap roller 39 may trickle down the gap roller 39, then enter the inside of the charging device 3 via the opening 33a, and then reach as far as the corona wire 32. In this state, moisture remaining in the charging device 3 permits excess current to flow therethrough, causing dielectric breakdown (leakage trace) of the surface of the photoconductive drum 2. This description deals with a case where the grid cleaner is fixed to the apparatus main body, and the charging device is made to slide. It should be understood, however, the same problem occurs when the grid cleaner is fitted to the charging device, and is then made to slide along it.